Windows to the Universe: Telescopes
A telescope is an optical instrument designed to make distant objects appear nearer, containing an arrangement of lenses, or of curved mirrors and lenses, by which rays of light are collected and focused and the resulting image magnified.
Types of Optical Telescopes
1.Refracting Telescope:
Uses a large objective lens to gather and focus light.
Suffers from chromatic aberration, an effect where different colors of light are focused at slightly different points, causing color fringing.
2.Reflecting Telescope:
Uses a large, curved primary mirror to gather and focus light.
This is the design used for all large, modern research telescopes because mirrors can be made much larger than lenses and do not suffer from chromatic aberration.
Powers of a Telescope
1. Light-Gathering Power: The most important power of a telescope. It is the ability to collect more light than the human eye, making faint objects visible. It is directly proportional to the area of the objective lens or primary mirror (and thus to the square of its diameter). A larger telescope collects more light.
2. Resolving Power (Resolution): The ability to distinguish between two closely spaced objects. Better resolving power means the telescope can see finer detail. It is also directly related to the diameter of the primary mirror/lens.
Magnification is a secondary and less important power, as any image can be magnified, but if the light-gathering and resolving power are poor, the magnified image will just be a larger blur.
Decoding Starlight: Spectroscopy
Spectroscopy is the technique of splitting light into its constituent wavelengths (a spectrum). By analyzing the spectrum of a star, astronomers can learn a tremendous amount about it.
A star's spectrum is an absorption spectrum: a continuous rainbow of colors with dark lines where specific wavelengths have been absorbed by elements in the star's atmosphere.
What a Spectrum Tells Us:
Chemical Composition: Every element has a unique 'fingerprint' of absorption lines. By matching the lines in a star's spectrum to known laboratory spectra, we can determine what elements the star is made of.
Temperature: The overall color or peak wavelength of a star's continuous spectrum reveals its surface temperature (Wien's Law). Hotter stars are blue, while cooler stars are red.
Radial Velocity (Motion): The Doppler effect shifts the position of the spectral lines.
If a star is moving towards us, its spectral lines are blueshifted (shifted to shorter wavelengths).
If a star is moving away from us, its spectral lines are redshifted (shifted to longer wavelengths). This is how Hubble discovered the expansion of the universe.